RDMA/efa: Use in-kernel offsetofend() to check field availability

Remove custom and duplicated variant of offsetofend().

Link: https://lore.kernel.org/r/20200310091438.248429-4-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>